An example of a known occupant weight detecting apparatus is disclosed in Japanese Laid-Open Publication No. 11(1999)-304579 published on Nov. 5, 1999. This occupant weight detecting apparatus is provided at the seat of a vehicle in which an airbag is equipped to protect the occupant on the seat. The apparatus is used to detect whether or not an occupant is seated on the seat or to control the gas generation volume for the expansion of the airbag corresponding to the occupant.
The occupant weight detecting apparatus is provided with four load sensors respectively mounted below the four corners of the seat to detect the load of the occupant. Based on the total value of the loads detected by these load sensors, the apparatus determines whether or not the occupant is on the seat or whether the occupant on the seat is an adult or a child. It is thus necessary to accurately detect the load applied on the seat.
Because the weight of the seat is applied to the load sensors, a zero point is provided to detect only the occupant weight. The occupant weight detecting apparatus thus detects an increasing load value from the zero point as the occupant weight.
However, the zero point of the load sensors may shift due to various use conditions of the seat. After a relatively long period of time has passed, a change or changes in the seat and the load sensors may occur. Such relatively long term change may include, by way of example, the distortion of the seat by its stress and the deterioration of the sensors. Due to such relatively long term change, the zero point may become shifted. Further, if the vehicle user places a cushion on the seat, the zero point may be shifted. If a shifting of the zero point occurs, it may be difficult to accurately detect the occupant weight on the seat.
A need exists for an occupant weight detecting apparatus which is not as susceptible to drawbacks such as those mentioned above.